Prism Sheet, a Back-Light Unit Using Said Prism Sheet, and a Transmission Type Liquid Crystal Display Device

ABSTRACT

[Objective] Providing a prism sheet, a back-light unit using the prism sheet, and a transmission type liquid crystal display device.  
     [Means to Solve Problems] The prism sheet according to the present invention is the prism sheet  10  for changing a direction of light propagation by reflecting light L launched with an intensity distribution of a predetermined angle range wherein: a reflective surface for perfect reflection of light L comprises a higher order surface  18  constructed by at least a small curvature surface and a surface different from the small curvature surface. Further, the present invention provides the back-light unit comprising the above-described prism sheet and the transmission type liquid crystal display device.

TECHNICAL FIELD

The present invention relates to a prism sheet having prism structures.More specifically, the present invention relates to a prism sheet havingprism structures for selectively reflecting light in a display device, aback-light unit comprising said prism sheet, and a transmission typeliquid crystal display device.

PRIOR ART

A liquid crystal display device, particularly a transmission type liquidcrystal display device is applied to a portable notebook personalcomputer, a television set and the like. In recent years, demands for alower power consumption, a longer battery life, a lighter weight, ahigher contrast, and a compactness have been increased. The transmissiontype liquid crystal display device normally comprises a back-light unitusing a light source such as a small fluorescent tube to irradiate lightfrom the rear side. It is said that this transmission type crystaldisplay device is capable of displaying various types of imagesaccording to changes in a light transmittance state of a liquid crystalpanel driven by a driving means such as a thin-film transistor array. Inthis case, when only power consumption of a light source such as a smallfluorescent tube is suppressed to specifically suppress powerconsumption and extend a battery life, a luminance of a display islowered resulting in a darkened display which is hard to be seen, and acontrast is also deteriorated. Consequently, it is difficult to provideexcellent display characteristics.

Accordingly, a back-light unit which is capable of suppressing powerconsumption together with improving a luminance has been demanded. Inrecent years, a back-light unit having a directional launching propertyis used commonly as a back-light unit of a liquid crystal displaydevice. The reason why is because a back-light having directionalitycauses a fewer loss by reflection than a conventional back-light of ascattering type, and this back-light unit is capable of effectivelyusing light from a small fluorescent tube to suppress power consumptionwhile providing an excellent display characteristic.

Further, back-light units employing various systems, for example, adirect-under type and a side-light type, have been proposed asback-light units for the liquid crystal display device. In recent years,the side-light type has become a mainstream along with increasingreduction of a thickness and a weight of a transmission type liquidcrystal display device.

FIG. 26 shows a conventional back-light unit of a side-light type. Asshown in FIG. 26, the conventional back-light unit comprises a lightsource 60 such as a small fluorescent tube, a light source section 62comprising a lamp holder having a reflector, a planer light guide 64which guides light from the light source 60 to inside of the surface,and a passive reflector 66 disposed under the planer light guide 64.Further, as shown in FIG. 26, optical components such as a prism sheet68 for light condensing and diffusing plate 70 for resolving unevenbrightness are disposed above the planer light guide 64.

Light irradiated from the light source 60 is condensed in the lightsource section 62 and thereafter launched onto the planer light guide64. The planer light guide 64, formed of a transparent acrylic resin orthe like, constitutes a planer light source which reflects and guidesthe incident light along the inside of the planer light guide 64 andprovides almost even brightness all over the planer light guide. Thereare two major methods for extracting light from the planer light guide,and an exit distribution depends on a method of extracting light.

One method of extracting light from the planer light guide 64 is the oneused commonly for a liquid crystal television set or a liquid crystalmonitoring device. This method extracts light L to the front side in anormal direction of the planer light guide 64 by scattering the light Lwith a dot pattern printed on the bottom of the planer light guide. Thelight L extracted by this method from the planer light guide 64 has arelatively isotropic distribution.

Another method for extracting light from the planer light guide 64 isthe one commonly used for a portable notebook personal computer, atelevision set, a cellular phone and the like. This method extractslight out of the planer light guide by constructing the planer lightguide 64 into a wedge shape to break perfect reflection conditions oflight. In this case, a dot pattern is also printed or processed on thebottom of the planer light guide to resolve deflection or unevenbrightness. When the method is employed to extract light, exit light hasa directional distribution at an intensity peak in an oblique directionwhich inclines greatly from a normal direction of the planer light guide64 to the direction of propagation of the light. In either of theextracting methods, light leaking from the bottom of the planer lightguide 64 is deflected to a normal direction by the passive reflector 62disposed on the rear side of the planer light guide.

Further, as shown in FIG. 26, one diffusing plate is or two diffusingplates are often disposed on the planer light guide to reduce unevennessof brightness of a light source. Still further, the required number ofprism sheets are appropriately disposed in addition to the diffusingplate (diffusing plates) to improve the front brightness by deflectingand condensing light from the planer light guide 64 and the diffusingplate 70.

The prism sheets have hitherto been studied to improve various featuresof the transmission type liquid crystal display device. For example,exit light from the surface of the planer light guide of the back-lightunit of the side-light type having the directional launchingproperty-normally shows an exit light distribution at an intensity peakin an oblique direction on the opposite side of a launching direction ofthe light source from a normal direction as aforementioned. Opticalsheets such as prism sheets or lens sheets are normally used to deflectthe light from the planer light guide. Various studies have alsohitherto been made about shapes of prisms and structure of lenses on theoptical sheets. For example, an upward type prism sheet shown in FIG. 26having prisms on the upper surface thereof and a downward type prismsheet disclosed in Japanese Patent Laid-Open Publication No. 2000-89009having prisms on the bottom surface thereof have been proposed.

The prism sheet having prisms facing up, i.e., the liquid crystal panelside, hereinafter refers to the upward type prism sheet. The prism sheethaving prisms facing down, i.e., the planer light guide side hereinafterrefers to the downward type prism sheet. The upward type prism sheet andthe downward type prism sheet have different principles of lightdeflection and different structures.

Prisms on the upward type prism sheet of the commonest shape havecross-sectional shapes of right-angled equilateral triangles. Two ofsuch prism sheets are combined and disposed on the planer light guide,one of the prism sheets is disposed so that triangular prisms areparallel to the light source (to condense exit light to the paralleldirection to the fluorescent tube), and the other prism sheet isdisposed so that triangular prisms are vertical to the small fluorescenttube (to condense light in the direction thereof). The diffusing plateis normally disposed between or under the prism sheets to eliminateunevenness of brightness.

However, although the upward type prism sheet is appropriate fordeflecting light with a low directionality of a liquid crystal panel ora television set, it cannot be said that the upward type prism sheet isappropriate for deflecting light from a back-light unit using awedge-shaped planer light guide having a high directionality to a normaldirection. Further, prism sheets having prisms of pentagonalcross-sectional shapes have hitherto been disclosed as upward type prismsheets in Japanese Patent Laid-Open Publication Heisei Nos. 7-230002 and8-254606, and upward type prism sheets having prisms with curved sidesurfaces have hitherto been disclosed as upward type prism sheets inJapanese Patent Laid-Open Publication Heisei No. 7-2947606 and No.2000-347011. The objective of both the prism sheets is to improvebrightness in a normal direction by employing the principle ofrefraction to deflect light.

On the other hand, downward type prism sheets configured to improvebrightness of back-light units having directional launching propertiesare disclosed in Japanese Patent Laid-Open Publication Heisei No.11-84111 and No. 2000-89009.

Such downward type prism sheet disclosed in Japanese Patent Laid-OpenPublication Heisei No. 11-84111 or No. 2000-89009 has hitherto been usedso that tops of prisms are disposed adjacently to each other on theplaner light guide. Further, a diffusing plate might be disposed betweenthe prism sheet and the planer light guide. Downward type prism sheetshaving various shapes have been proposed. However, the prism sheet withprisms having cross-sectional shapes of equilateral triangles formedthereon is used most generally. Still further, it is said that adownward type prism sheet having a structure specifically effective toimprove brightness is an asymmetric prism sheet with prisms formedhorizontally a symmetric thereon. However, various problems have beenpointed out with regard to these known downward type prism sheets.

FIG. 27 is a schematic illustration of the commonest prism sheet withprisms having cross-sectional shapes of equilateral triangles formedthereon. It is an object of the prism sheet shown in FIG. 27 to improvethe front brightness by deflecting light launched from the planer lightguide to the upper surface of prisms. In the downward type prism sheetshown in FIG. 27, light L from the planer light guide or the diffusingplate is launched from the first side surface f of the prisms and thenlight L is reflected on the second side surface s and deflected to anormal direction. At this time, the peak angles of prisms are designedso that the brightness in a normal direction becomes the highest againstlight launched at an angle of an intensity peak of light launched withan intensity distribution within a predetermined angle range. However,since the second surface s is a planer surface, light other than thelight launched at an angle of an intensity peak is not deflectedeffectively to a normal direction, the brightness is not improvedsufficiently by light condensing.

Further, FIG. 28 shows an asymmetric prism sheet disclosed in JapanesePatent Laid-Open Publication Heisei No. 10-254371 which was proposed forthe purpose of improving brightness. The cross section of a prismportion of the asymmetric prism sheet shows an in equilateral triangle,angles thereof from a normal direction of the sheet surface on the firstsurface f and the second surface s being different. Sides f and s of theprism are formed obliquely so that exit light at a predetermined anglefrom the planer light guide is deflected to the approximate direction ofthe normal line by reflection on the second surfaces of the prism. Inaddition, the prism is designed so that light can be reflected twiceinside the second surface s and the first surface f, respectively, andsecondary light is deflected to the front side. Accordingly, thedownward type prism sheet disclosed in Japanese Patent Laid-OpenPublication Heisei No. 10-254371 generates secondary light in additionto main light to a normal direction and also contributes to reflectionfrom the generated secondary light. Therefore, this downward type prismsheet has an effect of improving brightness.

However, the asymmetric prism sheet disclosed in Japanese PatentLaid-Open Publication Heisei No. 10-254371 has the followingdisadvantages: (1) since the cross-sectional shape of the side is asimple straight line, light deflected to a normal direction is lighthaving only two angles and exit light to a direction other than theseangles is not deflected, (2) a peak angle of light from a planer lightguide of a back-light having directional launching property ranges from60 to 80 degrees, when prisms are designed asynchronously facing down sothat light having such angle distribution is deflected to a normaldirection however, a vertical angle of a prism becomes no more than 50degrees and this sharp top causes an inferior productivity resulting inbad yield rate inappropriate for mass production.

As described above, while there is a tendency showing that back-lightunits having a high directionality are beginning to be used in recentyears, the upward prism sheet which has hitherto been used mostgenerally is disadvantageously inappropriate for deflecting andcondensing light having directionality by a reflection loss.

Further, regarding the downward type prism sheet, higher efficiency oflight reflection and condensing, easier manufacturing, and higher lightdirectionality have been expected.

In addition, a back-light unit capable of suppressing power consumptionof the small fluorescent tube and providing high brightness and highcontrast display under the conditions of compactness and lightweight hasbeen required.

Further, a compact and lightweight transmission type liquid crystaldisplay device capable of extending a battery driving time has beenrequired.

PROBLEMS TO BE SOLVED BY THE INVENTION

The present invention was made in consideration of the conventionaldisadvantages, and the objective thereof is to provide a prism sheethaving a high reflection efficiency, easy manufacturability and afunction of improving directionality of light.

Further, it is an object of the present invention to provide aback-light unit and a transmission type liquid crystal display devicecapable of enabling high brightness, high contrast, low powerconsumption, long battery driving time and compactness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a portion of the prism sheetaccording to the present invention.

FIG. 2 is a view showing the prism structure formed onto the prism sheetaccording to the present invention in details.

FIG. 3 is a cross-sectional view along with the pitch direction of theprism structure shown in FIGS. 1 and 2.

FIG. 4 is a perspective view showing the back-light unit according tothe present invention using the prism sheets shown in FIGS. 1 to 3.

FIG. 5 is a schematic cross-sectional view showing the back-light unitaccording to the present invention shown in FIG. 4.

FIG. 6 is a cross-sectional view showing the transmission type liquidcrystal display device comprising the back-light unit shown in FIGS. 4and 5.

FIG. 7 is an exploded perspective view showing the transmission typeliquid crystal display device according to the present invention indetails.

FIG. 8 is a view showing a typical distribution characteristic of exitlight from the planer light guide.

FIG. 9 is a view showing deflection of light by directions of prismstructures on the planer light guide having high directionality.

FIG. 10 is a graph showing a relationship between a refractive index anda vertical angle required to direct light at 77 degrees to the frontside.

FIG. 11 is a graph showing a relationship between a refractive index anda vertical angle in a downward type prism structure shown in FIG. 9(b).

FIG. 12 is a view showing a relationship between parameters used for thepresent invention.

FIG. 13 is a view showing a result of estimating transmittance of adownward prism sheet by employing a representative refractive index ofan acrylic resin 1.521.

FIG. 14 is a view showing a result of estimating transmittance of thedownward prism sheet by employing a representative refractive index ofan acrylic resin 1.521.

FIG. 15 is a view showing an action of light when prism structures areformed of triangles.

FIG. 16 is a view showing parameters used for the present invention.

FIG. 17 is a view showing a relationship between a point A and a point Bon side surfaces f and s of a prism structure, and angles.

FIG. 18 is a view showing a relationship between Bcx and α calculatedaccording to the present invention.

FIG. 18 is a view showing a relationship between Bcx and α calculatedaccording to the present invention.

FIG. 20 is a view showing a result of simulating a relationship betweenθ and β when α is 17 degrees.

FIG. 21 is a view showing design steps for a specific curved lineportion according to the present invention.

FIG. 22 is a view showing a residual between a curved line portion for alight source having an exit light characteristic shown in FIG. 8 and anapproximate curved line based on an ideal curved line and arcapproximation in a case of a prism structure having a vertical angle of55 degrees.

FIG. 23 is a view showing simulation of a transmitted light intensityassuming that a horizontal axis is a vertical angle of a prism structureand a vertical angle is the light intensity (W) at each visual angle.

FIG. 24 is a view showing a result of comparing an exit distributionwith that of a conventional prism sheet under the conditions shown inFIG. 23.

FIG. 25 is a view showing a prism structure formed with a pitch of 50μm.

FIG. 26 is a view showing a conventional back-light unit of a side-lightsystem.

FIG. 27 is a view showing the commonest structure having across-sectional shape of an equilateral triangle of a conventionaldownward type prism sheet.

MEANS TO SOLVE PROBLEMS

The present invention was made by adopting a downward prism structurewhile providing high light reflection and deflection efficiencies to aprism sheet by adopting a prism sheet employing perfect reflection froma higher order surface and further finding out that the prism sheet maybe manufactured easily at a low cost.

That is to say, according to the present invention, a prism sheet forchanging a direction of propagation of light propagation by reflectinglight launched with an intensity distribution of a predetermined anglerange, said prism sheet comprising: a reflective surface extending froma sheet base to a top for providing perfect reflection, the reflectivesurface comprising a higher order surface constructed by at least asmall curvature surface and a surface different from the small curvaturesurface.

In the prism sheet according to the present invention, the higher ordersurface comprises a plurality of continuous planer surfaces.

Further, in the prism sheet according to the present invention, thehigher order surface comprises at least a planer surface as the smallcurvature surface and a curved surface continuous thereto. The prismsheet according to the present invention comprises a plurality of thereflective surfaces formed in close proximity, and each closely disposedreflective surface being connected through a connection plane extendingfrom said sheet base to said top of an adjacent reflective surface. Inthe prism sheet according to the present invention, the small curvaturesurface extends to a predetermined position from the top of thereflective surface along with a direction toward the sheet base. Theconnection plane according to the present invention transmits lightlaunched with the intensity distribution of the predetermined anglerange to the reflective surface. The prism sheet according to thepresent invention preferably has a transmittance of no less than 90%,and an angle distribution of exit light is preferably no more than 15degrees at a full width at half maximum.

Further, the present invention provides a back-light unit comprising:

a light source for providing light exposed to the liquid crystal panel,

a planer light guide for changing a direction of propagation of thelight to the liquid crystal panel, and

a prism sheet disposed adjacently to said planer light guide;

wherein the prism sheet comprises a reflective surface for providingperfect reflection of light launched with an intensity distribution of apredetermined angle range from the planer light guide; and

the reflective surface comprising a higher order surface constructed byat least a small curvature surface and a surface different from saidsmall curvature surface.

The higher order surface of the back-light unit according to the presentinvention comprises a plurality of continuous planes. The higher ordersurface of the back-light unit according to the present inventioncomprises at least a planer surface as the small curvature surface and acurved surface continuous thereto. The prism sheet according to thepresent invention comprises a plurality of the reflective surfacesformed in close proximity, and each closely disposed reflective surfacebeing connected through a connection plane extending from the sheet baseto the top of an adjacent reflective surface. The small curvaturesurface preferably extends to a predetermined position from the top ofthe reflective surface along with a direction toward the sheet base. Theconnection plane according to the present invention transmits lightlaunched with the intensity distribution of the predetermined anglerange to the reflective surface. The top of the reflective surfaceconstituting the prism sheet according to the present invention isdisposed adjacently to the planer light guide.

Further, the present invention provides a transmission type liquidcrystal display device comprising a back-light unit and a liquid crystalpanel, the back-light unit comprising:

a light source for providing light exposed to said liquid crystal panel,

a planer light guide for changing a direction of propagation of thelight to the liquid crystal panel, and

a prism sheet disposed adjacently to the planer light guide;

wherein the prism sheet comprises a reflective surface extending from asheet base to a top for providing perfect reflection and the reflectivesurface comprises a higher order surface constructed by at least a smallcurvature surface and a surface different from the small curvaturesurface.

The higher order surface of the transmission type liquid crystal displaydevice according to the present invention comprises a plurality ofcontinuous planer surfaces. The higher order surface according to thepresent invention comprises at least a planer surface as the smallcurvature surface and a curved surface continuous thereto. The prismsheet according to the present invention comprises a plurality of saidreflective surfaces formed in close proximity, and said each closelydisposed reflective surface being connected through a connection planeextending from said sheet base to said top of an adjacent reflectivesurface. The small curvature surface according to the present inventiontransmits said light launched with the intensity distribution of thepredetermined angle range to the reflective surface. The top of thereflective surface constituting the prism sheet is disposed adjacentlyto the planer light guide. Further, according to the present invention,a lens element may be disposed between the back-light unit and theliquid crystal panel.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described with reference to but notlimited to specific embodiments shown in the drawings.

Section 1: Structure of Prism Sheet

FIG. 1 is a perspective view of a part of a prism sheet 10 according tothe present invention. The prism sheet 10 according to the presentinvention comprises a sheet base 12 and a prism structure 14 formeduniformly with the sheet base 12. The prism structure 14 extends allover the surface of the prism sheet 10 together with being disposedcontinuously at predetermined pitches p.

In order to prevent the pitches p (i.e., sizes of prisms) between prismstructures for the liquid crystal display device from being conspicuouswhen a screen is seen while preventing interference with a repetitivepattern with regularity such as pixels, sizes of the pitches p arepreferably made exactly the same as pitches between pixels and thepitches p are preferably disposed with positions thereof adjusted tothose of pixels or prisms are preferably created at sufficiently finepitches in comparison to those of pixels. However, since there isactually a technical difficulty in adjusting positions of pixels tothose of prisms minutely, prism structures are often created at finerpitches p than pitches between pixels.

However, since fineness of the liquid crystal display device has beenimproved in recent years, when the pitches p are made too fine, aninterference color may be generated depending on coherency (coherentlength) of a light source. In general, a pitch p is often set to about10 μm, specifically, in the range of 30 μm to 0.5 μm. In a specificembodiment of the present invention, a pitch p of the prism structuremay be set to 50 μm to 60 μm. Further, a fluctuation may be given to thepitches p of the prism structures according to the present invention toprevent interference.

As shown in FIG. 1, the prism structure is formed by the sheet base 12,a connection plane 16 and a higher order surface 18 constituting a neartriangular prism. The connection plane 16 connects the sheet base 12 anda top 18 a of the higher order surface 18. The higher order surface 18extends from the top 18 a to an end portion 20 of an adjacent connectionplane 16 b toward the sheet base 12. Light L is launched with anintensity distribution of a predetermined angle range to the connectionplane 16. The launched light L propagates to the higher order surface 18and thereafter the higher order surface 18 reflects the light L to thesheet base 12 side.

FIG. 2 shows only one of continuous prism structures to show a prismstructure formed on the prism sheet 10 according to the presentinvention in further details. As shown in FIG. 2, the prism sheet 10according to the present invention is formed of a light transparentmaterial and the constitution thereof allows launched light L to bereflected on the inner side of the higher order surface 18 of the prismstructure. The higher order surface 18 extends from the top 18 a for apredetermined length. The higher order surface 18 according to thepresent invention comprises a planer surface 22 as a small curvaturesurface and a curved surface 24 continuous to and starting from an endportion 22 a of the planer surface 22. The launched light L is reflectedby the planer surface 22 or the curved surface 24 to the sheet base 12,transmits the sheet base 12 and irradiated onto a diffusion sheet or aliquid crystal panel (not shown). The planer surface 22 may beenumerated as the above-described small curvature surface used for thepresent invention in a specific embodiment. However, the small curvaturesurface according to the present invention is not limited to the planersurface 22 and any geometrical surface, any a spherical surface or anycompound surface as long as the surface has a smaller curvature thanthat of the curved surface 24 continuous to the planer surface 22 and acurvature capable of reflecting light with a predetermined intensitypeak launched at an angle as small as no more than 15 degreesefficiently. In the present invention, since a planer surface may beused as a preferable embodiment of the above-described small curvaturesurface from viewpoints of manufacturing and designing, the presentinvention will be described on premise of using the planer surface 22 asa small curvature surface for convenience of explanation.

FIG. 3 is a cross-sectional view along with a direction of the pitch ofthe prism structure shown in FIG. 2. FIG. 3(a) shows a first embodimentof the higher order surface 18 included in the prism structure used inthe present invention. FIG. 3(b) shows a second embodiment of the higherorder surface 18 included in the prism structure. FIG. 3(c) shows athird embodiment of the higher order surface 18 included in the prismstructure. FIG. 3(a) shows the most preferable embodiment wherein thehigher order surface comprises the planer surface 22 and the curvedsurface 24 connected thereto. FIGS. 3(b) and 3(c) show higher ordersurfaces formed approximately of minute planes from the viewpoint ofmanufacturing.

As shown in FIG. 3(a), the higher order surface 18 included in the prismstructure used in the present invention comprises the planer surface 22extending from the top 18 a for a predetermined length and the curvedsurface 24 continuous thereto. The position along with the direction ofpitches from the top 18 a to the end portion 22 a of the planer surface22 extends to the critical position. The prism structure immediatelybefore this critical position prevents light at an intensity peak withinan angle distribution of launched light from reaching the criticalposition. Further, the planer surface 22 is formed in an angle allowingreflection of the light at the intensity peak within the angledistribution of the launched light to the normal direction mostefficiently. As shown in FIG. 3(a), light L1 launched at a greater anglethan that of the light at the intensity peak is launched to the sheetbase 12 side rather than to the end portion 22 a and then reflected onthe curved surface 24. Symbol d shown in FIG. 3(a) represents a distancefrom an x coordinate of the top of the prism structure to an xcoordinate of the end portion 22 a.

Upon assumption of a case wherein the planer surface 22 is extended tothe sheet base 12 side, since the planer surface 22 is optimized toreflect light launched at an intensity peak angle vertically to thesheet base 12 side, the planer surface 22 cannot reflect light launchedto the sheet base 12 side vertically to the sheet base 12 side anybetter than the end portion 22 a. Accordingly, in the most preferableembodiment of the present invention, an upper portion of the end portion22 a is formed as the curved surface 24 wherein a reflection anglethereof against launched light is more increased continuously on thesheet base 12 than on the end portion 22 a.

Further, in another embodiment of the present invention, the curvedsurface 24 may be formed so that a curvature changes continuously asshown in FIG. 3(a). The curved surface 24 may also be formed of an arcapproximating to the curved surface 24 or a plurality of planer surfacesfrom the viewpoints of manufacturability and cost. FIG. 3(b) is a viewshowing the second embodiment wherein the higher surface 18 is formed ofa plurality of planer surfaces. In the embodiment shown in FIG. 3(b),the planer surface 22 is also constituted in the same manner as thatdescribed in FIG. 3(a). Further, in the embodiment shown in FIG. 3(b),the curved surface 24 continuous to the planer surface 22 is replaced bya plurality of minute planer surfaces 24 a having shorter lengthsconstituted to have an angle of β>γ according to an angle of launchedlight L1.

When the curved surface 24 is approximated with minute planer surfacesin the present invention, any number of minute planer surfaces may beused to approximate the curved surface 24. However, the number of minuteplaner surfaces is preferably large as possible for good approximationof the curved surface 24. FIG. 3(c) is a view showing an embodimentwherein the above-described curved surface 24 is approximated with twominute planer surfaces 24 a and 24 b. In this case, the minute planersurfaces are also constituted so that each planer surface may have anangle of β>γ1>β>γ2. In general when the curved surface 24 is furtherapproximated with m minute planer surfaces in the present invention,each angle may be determined to satisfy the relationship of β>γ1>β>γ2> .. . γ_(m−1)>γ_(m).

Further, in the prism structure shown in FIG. 3, α may be 0 to 50degrees, more preferably 25 to 40 degrees and the vertical angle δ ofthe prism may be 39 to 90 degrees, preferably 50 to 80 degrees, morepreferably 62 to 77 degrees.

Section 2: Back-Light Unit and Transmission Type Liquid Crystal DisplayDevice of Present Invention

FIG. 4 is a perspective view of a back-light unit 26 according to thepresent invention using the prism sheet 10 shown in FIGS. 1 to 3. Theback-light unit 26 shown in FIG. 4 comprises a planer light guide 30 forguiding light from a light source 28 to irradiate the light onto aliquid crystal panel (not shown), a reflective sheet 32 for reflectinglight to the liquid crystal panel with a high efficiency, the prismsheet 10 according to the present invention disposed adjacently to theplaner light guide 30 and a diffusing plate 34 disposed adjacently tothe prism sheet 10 on the liquid crystal panel side.

The diffusing plate 34 is not necessarily disposed on the position shownin FIG. 4 and may be used appropriately with another constitution inconsideration of an optical characteristic. Further, the light source 28of the back-light unit 26 shown in FIG. 4 comprises a fluorescent tubesuch as a small fluorescent tube and constitutes the back-light unit 26uniformly by being accommodated into the accommodation portion 36constituting a reflector. In addition, any small light source other thanthe small fluorescent tube may be used as long as the light source iscapable of providing an appropriate spectrum characteristic. Light fromthe light source is launched from the planer light guide 30 andthereafter deflected to the normal direction of the planer light guide30, i.e., the direction represented by an arrow A in FIG. 4 and thenirradiated onto the liquid crystal panel.

FIG. 5 is a schematic cross-sectional view of the back-light unitaccording to the present invention shown in FIG. 4. The cross-sectionalview shown in FIG. 5 specifically shows a relationship between the prismsheet 10 and light irradiated from the planer,light guide 30. As shownin FIG. 5, light led into the planer light guide 30 is given an angulardirectionality while being reflected in the inside of the planer lightguide 30 and then irradiated onto the prism sheet 10. In the back-lightunit 26 of this application, the prism sheet 10 raises thedirectionality of the launched light to realize high brightness to thenormal direction of the planer light guide 30.

In the present invention, an angle of light launched to the prism sheet10 refers to an angle measured counterclockwise from an upper surface 30a of the planer light guide 30. As shown in FIG. 5, light launched fromthe planer light guide 30 to the prism sheet 10 is reflected by thehigher order surface 18 of prism structures formed on the prism sheet10, transmitted through the diffusing plate 34 and directed to theliquid crystal panel (not shown). FIG. 5 enumerates specific lighthaving an angle corresponding to an intensity peak of exit light fromthe planer light guide 30 as light launched from the planer light guide30 to the prism sheet 10. FIG. 5 shows that light L is reflected on theend portion 22 a of the higher order surface 18 and then the directionthereof is changed to the direction toward the liquid crystal panel.

Further, as shown in FIGS. 1 to 3, a base 12 side of the higher ordersurface 18 is formed of curved surfaces in a preferred embodiment of thepresent invention. By thus adopting the higher order surface 18 to thepresent invention, light from the planer light guide 30 may be reflectedso that even light launched beyond the end portion 22 a at a largerangle than that of the light shown in FIG. 5 to the sheet base 12 sideand reflected thereon is directed to the normal direction of the planerlight guide 30 efficiently against the liquid crystal panel.

FIG. 6 is a schematic cross-sectional view showing a transmission typeliquid crystal display device comprising the back-light unit 26 shown inFIGS. 4 and 5. The transmission type liquid crystal display device 38according to the present invention comprises the back-light unit 26, aliquid crystal panel 40 disposed adjacently to the back-light unit 26, alens element 40 a for condensing or enlarging light arradiated onto theliquid crystal panel 40, a pair of polarizing plates 42 a and 42 bdisposed adjacently to both sides of the liquid crystal panel anddiffusing plate 44 for diffusing light transmitted through the liquidcrystal panel 40. The lens element 40 a shown in FIG. 4 may be formedinto a convex lens or lenticular lens on a lens sheet or the like in thepresent invention. In addition, in the embodiment shown in FIG. 6 is thesame as the embodiment shown in FIG. 5 except that the diffusing plateshave different configurations.

FIG. 6 shows the lens element 40 a used for the transmission type liquidcrystal display device 38 according to the present invention formed intoa convex lens for each pixel between the liquid crystal panel 40 and theback-light unit 26. Further, according to the present invention,lenticular lenses may be disposed on opposite positions. By using theabove-described lens elements 40 a together for the transmission typeliquid crystal display device 38 according to the present inventionshown in FIG. 6, the effective opening rate of the transmission typeliquid crystal display device 38 maybe improved. Further, use of theabove-described lens elements 40 a is not essentially required for thepresent invention and the conventional transmission type liquid crystaldisplay device without using lens elements 40 a is capable of equallyenabling high brightness, longer battery driving time and high contrast.

Since the transmission type liquid crystal display device 38 accordingto the present invention shown in FIG. 6 comprises the back-light unit26 comprising the prism sheet 10 according to the present invention,light given by the light source 28 is irradiated onto the liquid crystalpanel 40 with a high efficiency and a high directionality. Accordingly,the transmission type liquid crystal display device 38 according to thepresent invention is capable of providing high brightness even when alight source of a low power consumption type is used and of a liquidcrystal display with high contrast.

FIG. 7 is an exploded perspective view showing the detailed structure ofthe transmission type liquid crystal display device 38 according to thepresent invention. The transmission type liquid crystal display device38 in the embodiment of the present invention shown in FIG. 7 comprisesan upper frame 48 which picturizes a display window 46 for picturizingan effective frame of the transmission type liquid crystal displaydevice 38, the back-light unit 26 according to the present invention andthe liquid crystal panel 40 disposed between the upper frame 48 and theback-light unit 26.

As shown in FIG. 7, the back-light unit 26 mounted on a lower case 50constitutes the transmission type liquid crystal display device 38 bybeing supported with the upper frame 48 in a united body. Since theabove-described back-light unit 26 uses the prism sheet 10 according tothe present invention, light from the light source 28 is irradiatedeffectively onto the liquid crystal panel 40 to make it possible toprovide a liquid crystal display with high brightness and high contrast.Further, by using the prism sheet 10 according to the present invention,the electric power applied to the light source to provide a liquidcrystal display of the same level as that of a conventional liquidcrystal display may be reduced or a light source consuming a fewelectric power may be used. Consequently, the transmission type liquidcrystal display device enabling energy consumption and a compact designis provided.

The design of the higher order surface 18 of the prism sheet 10according to the present invention will be described in details.

Section 3: Design of Higher Order Surface Adopted to Prism SheetAccording to Present Invention

FIG. 8 shows characteristics of an exit light distribution from theplaner light guide having directionality used in the representativetransmission type liquid crystal display device. Any exit lightcharacteristics may be used in principle for the planer light guideaccording to the present invention. In particular, exit lightcharacteristics may be applied effectively to the planer light guidehaving high directionality shown in FIG. 8. Further, a peak angle of theexit light characteristics is not limited to 77 degrees. A planer lightguide having any peak angle may be used as long as the peak angle may becoped with in consideration of manufacturability.

As shown in FIG. 8, exit light from the planer light guide used in aspecific embodiment of the present invention has an intensity tensilepeak at 77 degrees and the full width at half maximum of the exit lightis about 20 degrees. In addition, an angle of exit light from the planerlight guide described in FIG. 8 and thereunder is defined to be measuredclockwise assuming that the normal direction of the planer light guideis 0 degree and a direction to the light source is −90 degrees.

As shown in FIG. 8, exit light from the planer light guide is launchedwith a predetermined angle range and characterized by being aslantextremely against the normal direction of the planer light guide, i.e.,the direction to the liquid crystal panel. Further, FIG. 8 shows thatthe exit light has a narrow full width at half maximum of an angle andhigh directionality. In order to deflect light from the above-describedplaner light guide efficiently to the liquid crystal panel forcontribution to the brightness of the liquid crystal panel, the prismstructures of the prism sheet are preferably facing the planer lightguide side as described hereunder.

That is to say, a resin used to manufacture prisms in general is atransparent acrylic resin having a refractive index within a range of1.49 to 1.56. FIG. 9 is a view showing deflection of light depending ona direction of prism structures on the planer light guide having highdirectionality. According to the present invention, an angle of lightlaunched from the planer light guide is defined to θ and an angle of alight launched from a prism is defined to φ. FIG. 9 shows a layout ofprism structures corresponding to the angle definitions. In FIG. 9, FIG.9(a) shows an action of light when the prism structures are formed toface up and FIG. 9(b) shows an action of light when the prism structuresare formed to face down.

As shown in FIG. 9(a), when the prism structures are formed to face up,each prism structure is formed into an equilateral triangle with a topof the prism defined to δ and when the equilateral triangle is formed sothat the vertical angle thereof faces up, the bottom angle is (π−δ)/2.The relationship between the angles is represented by the followingformula: $\begin{matrix}{{{\sin\quad\theta_{1}} = {\frac{1}{n}\sin\quad\overset{\backprime}{\theta}}}{\theta_{2} = {\frac{\pi - \delta}{2} - \theta_{1}}}{{\sin\quad\theta_{3}} = {n\quad\sin\quad\theta_{2}}}{\phi = {\frac{\pi - \delta}{2} - \theta_{3}}}} & \left\lbrack {{Formula}\quad 1} \right\rbrack\end{matrix}$(wherein parameters represent the angles shown in FIG. 9(a)). FIG. 10 isa graph showing the relationship between a refraction index and avertical angle required to direct light at 77 degrees to the front sidewhen the above formula is used. As shown in FIG. 10, in the range of therefraction index of the above-described acrylic resin, the verticalangle δ of an upward prism structure is limited to the range of 20 to 30degrees and the vertical angle is therefore extremely sharp.

The prism sheet adopting the layout shown in FIG. 9(a) has disadvantagesof extreme difficulty in creating a metal mold thereof, incompleteseparation of the top portion from the metal mold after the sheet isformed by employing the injection mold method or the compression moldmethod and a low yield rate. This means that efficiently directing lighthaving directionality to an upper direction in the figure, i.e., thedirection to the liquid crystal panel, is actually impossible. Theabove-described conditions also apply to asymmetric triangles other thanthe symmetric triangle shown in FIG. 9(a).

On the other hand, when an equilateral triangle of a downward prismstructure having a vertical angle of δ is considered in the same manner,the relationship between the angles shown in FIG. 9(b) is represented bythe following formula: $\begin{matrix}{{\theta_{1} = {\theta - \frac{\pi}{2} + \frac{\delta}{2}}}{{\sin\quad\theta_{2}} = {\frac{1}{n}\sin\quad\theta_{1}}}{\theta_{3} = {{\frac{3}{2}\delta} - \theta_{2} - \frac{\pi}{2}}}{{\sin\quad\phi} = {n\quad\sin\quad\theta_{3}}}} & \left\lbrack {{Formula}\quad 2} \right\rbrack\end{matrix}$

FIG. 11 is a graph showing a relationship between a refractive index anda vertical angle of the downward prism structure shown in FIG. 9(b). Thegraph shown in FIG. 11 shows that the vertical angle of the prismstructure may be increased to 60 to 70 degrees when light is reflectedon a side r of the prism for deflection. Accordingly, when a planerlight guide providing exit light having high directionality is used, itis effective to form prism structures facing down and, at the same time,light is reflected on side surfaces r of the prisms.

Further, in consideration of a transmittance with reference to FIG. 9,when prism structures are formed to face up, light needs to be refractedtwice and an overall transmittance is determined by a product oftransmittance values at interfaces. When prism structures are formed toface down, there are two refractive interfaces as those of the upwardtype prism structures and FIG. 9(b) shows addition of one reflectivesurface to the prism structures. According to the present invention, areflectance may be made 100% by setting angles of the prism structuresso that the reflective surface r shown in FIG. 9(b) reflects launchedlight perfectly. Consequently, the efficiency may be prevented fromdeterioration caused by addition of reflection.

FIGS. 13 and 14 show results of estimating the transmittance of thedownward type prism sheet by employing a refractive index of 1.521 of arepresentative acrylic resin. FIG. 13 shows a case wherein lightpropagates from atmospheric air into a medium. FIG. 14 shows a casewherein light propagates from a medium into atmospheric air. When lightpropagates from a medium into atmospheric air, it is known that thelight is reflected perfectly and the reflectance is 100% at a largerangle than the angle (critical angle) determined by the formula below.When the refractive index of 1.521 is used, the critical angle given bythe formula below is 41 degrees. When prism structures are formed toface down, the prism plane may be configured so that reflection thereonalways satisfies the perfect reflection condition and no extremedisadvantage is therefore caused when an acrylic resin is used inparticular. $\begin{matrix}{{\theta\quad c} = {\arcsin\left( \frac{1}{n} \right)}} & \left\lbrack {{Formula}\quad 3} \right\rbrack\end{matrix}$

A shape of a prism structure will be studied. In general, a prismstructure having a triangle cross-sectional shape has hitherto beenused. In this case, various shapes of triangles are adopted, forexample, an equilateral triangle and asymmetric triangle have beenproposed as aforementioned. As aforementioned, use of the prism sheetcomprising prisms having polygonal cross-sectional shapes or prismshaving sides formed of curved lines have hitherto been enumerated.However, any of these prism sheets is capable of adopting a structurefor perfect reflection and cannot deflect launched light having a peakin an oblique direction (60 to 80 degrees) to the normal direction ofthe planer light guide, i.e., upper side of the figure.

The present inventors were led to the present invention from finding outthat light from a light source such as a planer light guide havingdirectionality may be deflected efficiently to the front side bycombining straight lines and curved lines. By this combination surfacesforming prism structures in a pair with other sides formed of straightlines are formed into high order surfaces to use the perfect reflection.The prism sheet according to the present invention will be describedbased on the principle of the present invention while taking intogeometric consideration of prism structures.

In order to deflect light from the planer light guide efficiently to thenormal direction of the planer light guide toward the liquid crystalpanel side, an angle of an oblique surface is determined according to apeak angle of an exit light from the planer light guide for triangularor polygonal prisms. FIG. 15 is a view showing an action of light whenprism structures are formed of triangles. Light L with a peak angle ofexit light from the planer light guide is launched to prisms while beingrefracted on the first surface f and then deflected to the normaldirection after being reflected perfectly on the second surface s. Sinceprism structures of the same shapes are disposed in parallel at pitchesp, light L at an angle of an intensity peak is reflected by only aneffective area ea of the second surface s and then deflected to thefront side. Accordingly, light at a smaller exit angle than the peakangle, i.e., light launched at a larger launching angle for prismstructures is reflected above the effective area ea shown in FIG. 15.

The end of the upper side of the figure of the effective area eaconstitutes the end portion 22 a shown in FIG. 3. This end portion 22 ais hereinafter defined to a connection point Bc in this section.

According to the present invention, when a surface other than a planersurface 22 is adopted as a small curvature surface, the connection pointBc may be defined as a position whereon light L at an intensity peak oflight launched with a predetermined angle distribution is reflected bythe higher order surface 18. Further, from a viewpoint of practical use,a connection point Bc may be selected appropriately as a point on thehigher order surface 18 in the vicinity of the connection point Bcwithin the range of reflecting light within a range of full width athalf maximum (FWHM) of an intensity peak in angle distribution oflaunched light. In the present invention, the x coordinate of theconnection point Bc is hereinafter refers to as Bcx and the y coordinateis hereinafter refers to Bcy. The light from the connection point Bc tothe upper side of the figure indicated by an arrow L1 is launched to adirection shifted from the normal direction as indicated by an arrow Bin FIG. 15 because the second surface s is optimized for the peak anglewhen the present invention is not applied. The same condition is alsoapplicable when polygons are adopted simply as prism structures.Accordingly, it can be said that prism structures having reflectivesurfaces formed of only simple straight lines cannot provide asufficient efficiency.

Further, it can be thought that the entire second surface s is formed ofa curved line. However, peak angles of exit light from the planer lightguide cannot be all directed to the normal direction and the overallreflection efficiency is therefore deteriorated instead. In order todeflect the light having high directionality shown in FIG. 8 efficientlyto the normal direction, it is found out by the present invention thatforming the higher order surface of straight lines and curved linesallows the most effective use of light from the light source. That is tosay, since light L at a peak angle from the light source is thrown ontoonly an effective area ea, the surface is formed of a straight line atan angle optimized to the peak angle in the effective area ea, and, onthe other hand, the angle of the surface is preferably changeddynamically to face the front side as indicated by the curved surfaceshown by the dashed line in FIG. 15.

In the upper portion than the connection point Bc in FIG. 15, an angleof exit light allowing the highest brightness is a little bit smallerangle than the peak angle, and the intensity of the exit light reducessmoothly as the same is being separated farther from the peak angle.Therefore, the upper portion than the connection point Bc is preferablyformed into a curved line having an angle changing continuously inprinciple. However, the present invention makes it possible toapproximate the curved line with a plurality of planer surfaces andarcs. In addition, the specific embodiment for approximating the curvedline with arcs will be described in more details.

A design of prism structures comprising higher order surfaces adopted tothe present invention will be described in details. Parameters used todesign prism structures comprising the higher order surfaces are a peakangle (θ peak) of light from the planer light guide, refractive indicesof prisms (n) and vertical angles of prisms. A vertical angle of a prismis defined assuming that an angle of an oblique surface (first sidesurface f) of a side whereto light is launched against the normal lineis α and an angle of an oblique surface on the opposite side (a surfacecomprising straight lines and curved lines; second side surface s) is β.FIG. 16 and the following table show all the parameters used for thepresent invention. TABLE 1 [Parameters Used for Designing PrismStructures] Pitch: pitch Height: height Refractive index of material: nAngle of launched light: θ (angle from the normal direction) Peak angleof launched light: θ peak Angle of launching side surface α of verticalangle: Angle of opposite surface of β₀ vertical angle: Angle of curvedline on point B: β (continuously changes against θ) Origin (0, 0): Topof adjacent left prism structure Difference between connection d pointBc and prism top:

As shown in FIG. 16, a point A is a point whereon launched light passesthrough the first side surface f and a point B is a point whereon lightwhich has passed through the first side surface f is reflected on thesecond side surface s. Further, the connection point of a planer surfaceportion and a curved line portion is represented by Bc in FIG. 16.

As shown in FIG. 16, when the prism sheet according to the presentinvention is designed, angles of tops of prisms are separately definedto α and β. Normally, β may be determined uniquely against predeterminedα from the relationship represented by the following formula.

Coordinates of point A: $\begin{matrix}\left\{ \begin{matrix}{{{Formula}\quad{of}\quad{light}\quad y} = \frac{x}{\tan\quad\theta_{peak}}} \\{{{Formula}\quad{of}\quad{prism}\quad{surface}\quad y} = {\frac{- 1}{\tan\quad\alpha}\left( {x - {pitch}} \right)}}\end{matrix} \right. & \left\lbrack {{Formula}\quad 4} \right\rbrack\end{matrix}$

By resolving the above simultaneous equations, the position coordinateof the point A is determined as the following formula: $\begin{matrix}{{{{Ax} = \frac{{pitch}*\tan\quad\theta_{peak}}{{\tan\quad\alpha} + {\tan\quad\theta_{peak}}}},\quad{{Ay} = \frac{pitch}{{\tan\quad\alpha} + {\tan\quad\theta_{peak}}}}}\left( {{pitch} = {{Bcx} - d}} \right)} & \left\lbrack {{Formula}\quad 5} \right\rbrack\end{matrix}$

Coordinates of point Bc:

Light refracted on the point A propagates through a medium and an anglethereof is given by the following formula from the Snell's law:$\begin{matrix}{{\sin\quad\phi_{1}} = {n*\sin\quad{\phi_{2}\left( {\phi_{1} = {\theta - \frac{\pi}{2} + \alpha}} \right)}}} & \left\lbrack {{Formula}\quad 6} \right\rbrack\end{matrix}$

Since light reflected on the high-order surfaces on the point B isdirected to the normal direction of the planer light guide, thecondition of the following formula need be satisfied: $\begin{matrix}{{\frac{\pi}{2} + \phi_{2} - \alpha} = {2\beta}} & \left\lbrack {{Formula}\quad 7} \right\rbrack\end{matrix}$

By the above formula, β is determined uniquely by α. In addition, FIG.17 shows the relationship between the above-described points A and B andthe angles.

The point Bc corresponds to the point B determined according to θ peakand the coordinates thereof may be obtained as resolutions of thefollowing simultaneous equations: $\begin{matrix}\left\{ \begin{matrix}{y = {\frac{1}{\tan\quad\beta}\left( {x - {pitch}} \right)}} \\{y = {{{\tan\left( {\alpha - \phi_{2}} \right)}\left( {x - {Ax}} \right)} + {Ay}}}\end{matrix} \right. & \left\lbrack {{Formula}\quad 8} \right\rbrack\end{matrix}$and given by the following equation: $\begin{matrix}{{{{Bcx} = \frac{\frac{pitch}{\tan\quad\beta} + {{Ax}*{\tan\left( {\alpha - \phi_{2}} \right)}} + {Ay}}{\frac{1}{\tan\quad\beta} - {\tan\left( {\alpha - \phi_{2}} \right)}}},{{Bcy} = \frac{{\left( {{Ax} + {pitch}} \right){\tan\left( {\alpha - \phi_{2}} \right)}} + {Ay}}{1 - {\tan\quad\beta\quad{\tan\left( {\alpha - \phi_{2}} \right)}}}}}\left( {{pitch} = {{Bcx} - d}} \right)} & \left\lbrack {{Formula}\quad 9} \right\rbrack\end{matrix}$

The relationship between α and β at θ peak will be described in moredetails.

FIG. 18 shows the relationship between calculated Bcx and α. FIG. 19shows the relationship between calculated Bcy and α. Coordinates of theconnection point Bc may be defined from the above-describedrelationships. A curved surface or a curved line starting from theconnection point Bc may be defined as follows. Namely, exit light fromthe planer light guide may be determined by making the exit lightpropagate through a top of a prism, which is an origin of coordinateaxes, and changing β along with reduction of the launching angle of thelight from θ peak as shown in FIG. 8. That is to say, it is important toknow the relationship of β to θ (launching angle). From this β which isdetermined by α, it is known that α is a main parameter dominating ashape of a higher order surface used for the present invention.

FIG. 20 shows a result of simulating the relationship between θ and βwhen α is 17 degrees. As shown in FIG. 20, it is known that β need beincreased along with increase of the launching angle θ. Therefore, in apreferable embodiment of the present invention, a portion of the curvedline 24 may be defined to a curved surface having a curvature increasinggradually according to a launched light characteristic of the lightsource.

The design of the specific curved line portion according to the presentinvention may be achieved by adopting the following steps with referenceto FIG. 21. Primarily, define the point Bc to B₀ and then sequentiallydefine B₁, B₂, . . . .

(1) Determine α.

(2) Determine coordinates of the point Bc (B₀) against α. The top andthe point Bc are connected with a straight line.

(3) Obtain the relation between θ and β.

(4) Assume that a notch width is

θ, obtain φ₂ at θ−

θ and assume that an intersection of a straight line having aningredient of φ₂−α through the point B₀ and an ingredient π/(2−β) is anext point B₁.

(5) Thereafter, obtain the point B₂ based on the point B₁ in the samemanner and repeat the same operation.

According to the present invention, an ideal curved surface may befurther obtained by determining y sequentially by replacing θ peak by θin general. That is to say, the ideal curved surface according to thepresent invention may be obtained by determining γ sequentially so thatthe conditions of the following formula are satisfied: $\begin{matrix}{{{\gamma(\theta)} = {\frac{\pi}{4} - \frac{\alpha}{2} + {\frac{1}{2}{\phi(\theta)}}}}{{\phi(\theta)} = {\sin^{- 1}\left\{ {\frac{1}{n}{\sin\left( {\alpha + \theta - \frac{\pi}{2}} \right)}} \right\}}}} & \left\lbrack {{Formula}\quad 10} \right\rbrack\end{matrix}$

According to the present invention, the curved line portion may bedesigned by simply repeating the above-described procedures for therequired number of times. However, an ideal curved line may beapproximated with arcs or straight lines closest to the above-describedcurved line according to the present invention. Approximation of theabove-described curved line with arcs will be described specifically. Inthis case, since the curved line need be connected smoothly to thestraight line portion on the point B₀, the arcs are approximated withthe following parameters:

When an arc having a curvature center of (c₁, c₂) and a curvature radiusR is used, the relationship represented by the following formula issatisfied: $\begin{matrix}{\left( {c_{1},c_{2}} \right) = {{\frac{R}{\sqrt{{By}^{2} + \left( {{pitch} - {Bx}} \right)^{2}}}\left( {{By},{{pitch} - {Bx}}} \right)} + \left( {{Bx},{By}} \right)}} & \left\lbrack {{Formula}\quad 11} \right\rbrack\end{matrix}$

Therefore, the only independent parameter is R and approximation may beperformed with the arc formula represented by the following formula:(x−c ₁)²+(y−c)² =R ²   [Formula 12]

As a result of an enthusiastic study, the present inventors found outthat an equal efficiency in practical use to that of a higher ordersurface formed of ideal curved lines even when the curved line portionis approximated with arcs.

FIG. 22 shows a residual between the ideal curved line and theapproximate curved line based on arc approximation regarding the curvedline portion against the light source having the launched lightcharacteristic shown in FIG. 8 for a prism structure having a verticalangle of 55 degrees. As shown in FIG. 22, there is scarcely a residualbetween the both curved lines even when arcs are used for approximation,and fine approximation is possible. Further, according to the presentinvention, the ideal curved line may be approximated with a plurality ofminute planer surfaces as shown in FIGS. 3(b) and 3(c) although theefficiency somewhat deteriorates.

Section 4: Study of Simulation of Reflection Property of Prism SheetAccording to Present Invention

Shapes of prism structures obtained as aforementioned for the presentinvention were used for three-dimensional simulation of light tracing.

When the prism sheet is actually disposed in the liquid crystal displaydevice, the diffusing plate is mounted on a predetermined position abovethe prism sheet as a general configuration to prevent unnecessary moiré.For this purpose, the diffusing plate having an approximated diffusionproperty assuming that light was scattered by Gaussian scatter based ona really measured value was disposed as a model for simulation.

The light source was assumed to have an intensity with angulardistribution shown in FIG. 8. For the prism structure portion, a valuedesigned by reflecting properties of the light source was used as was,Further, the diffusing plate was disposed on the opposite side to thelight source beyond the prism sheet according to the present inventionfor simulation. Intensities of transmitted light with visual angles of±1 degree and ±2 degrees was simulated when vertical angles of prismstructures of the prism sheet according to the present invention werechanged. Still further, the downward type asymmetric prisms disclosed inJapanese Patent Laid-Open Publication Heisei No. 11-84111 and otherJapanese Patent Laid-Open Publications were also simulated forcomparison. FIG. 23 shows the results of the simulations.

FIG. 23 is a view showing simulation of an intensity of transmittedlight assuming that the horizontal axis is a vertical angle of a prismstructure and the vertical axis is the intensity of the light (W) ateach visual angle. Normally, in the transmission type liquid crystaldisplay device, front brightness is often measured at a visual angle of1 degree to 2 degrees. The straight line shows a result of simulating adownward type asymmetric prism adopted as an example of a conventionalprism. The curved line shows a result of simulating the prism sheetaccording to the present invention.

As shown in FIG. 23, when the planer light guide having the exitdistribution shown in FIG. 8 is used and a pitch between prismstructures is assumed to be 50 μm, the highest front brightness isobtained at a vertical angle of about 68 degrees and it is known thatthis property gives a sufficient manufacturability. Further, as shown inFIG. 23, when the prism sheet according to the present invention isadopted, brightness at a predetermined visual angle may be improved for1.2 to 1.5 times in comparison to that of the conventional prism sheet.

Further, FIG. 24 shows a result of comparing exit distribution under theconditions shown in FIG. 23. FIG. 24 shows that an intensitydistribution becomes about 25 degrees for the conventional downward typeasymmetric prisms at a full width at half maximum and about 12 degreesfor the prism structures according to the present invention when theprism sheet according to the present invention is used and a full widthat half maximum of an angle distribution of exit light is reduced tonear half according to the present invention. According to the presentinvention in particular, the full width at half maximum of the angledistribution of the exit light is preferably no more than 15 degreeswhen the transmittance of the prism sheet is no less than 90% so that anexcellent brightness characteristic maybe added. Further, FIG. 24 showsthat a peak intensity at a desired visual angle is improved at the sametime by applying the present invention and that light may be condensedefficiently and the front brightness may be improved efficientlyaccording to the present invention.

An embodiment of the transmission type liquid crystal display device 38shown in FIG. 6 according to the present invention will be studiedregarding the display characteristic provided by the present invention.In the transmission type liquid crystal display device 38 shown in FIG.6, a lens element 40 a is used. When the lens element 40 a shown in FIG.6 is used, a size of an image (1) is about 1=d×2 sin (θ/2). Here, drepresents a thickness of a glass substrate of the liquid crystal paneland θ represents a full width at half maximum of a spread angle of lightlaunched to the liquid crystal panel. When d=0.7 mm and θ=12 degrees,1≈150 μm.

Accordingly, when the prism sheet 10 according to the present inventionis adopted to the transmission type liquid crystal display device 38according to a specific embodiment of the present invention shown inFIG. 6, the prism sheet 10 according to the present invention may beimproved (angle distribution characteristic of exit light=full width athalf maximum of 12 degrees). Further, a liquid crystal panel 40 formedon the glass substrate having a thickness of 0.7 mm is assumed to beused, a transmittance of light of a liquid crystal panel having pixelseach sized no less than 150 μm may be improved. Still further, from thesimilar consideration, a glass substrate used frequently in recent yearshaving a thickness of 0.5 mm or less is effective for a liquid crystalpanel whereon further minutely pixels are formed. Furthermore, accordingto the present invention, since a spread angle range of light passingthrough a liquid crystal is narrow, disadvantages pointed out regardingto the conventional transmission type liquid crystal display device suchas contrast inversion which is a major problem of light passingobliquely through a display element of a TN liquid crystal or the likemay be improved.

FIG. 25 shows a prism structure formed with a pitch of 50 μm as aspecific embodiment of the present invention. As examples of methods ofcreating a prism having a size of tens of μm efficiently, the resinmolding method, the injection molding method, the compression moldingmethod, and the photopolymer method (hereinafter abbreviated to “2Pmethod”) are the most effective molding methods from the viewpoint ofeconomic efficiency. Any of these methods uses a metal mold made ofcopper, nickel or the like or a plastic mold made of an epoxy resin,acrylic resin or the like and forms a replica of a base type prism. Forthis purpose, a negative shape mold is used for a desired prism shape.

As a method of creating the above-described negative mold made of meal,a method of creating the mold by direct cutting or processing a resin ora metal by cutting into the positive shape and thereafter negativenickel replica made by electroforming or the like may be enumerated.Further, when a positive shape may be created with a metal or a resinand thereafter the positive shape may be transcribed to a resin tocreate a negative resin mold. That is to say, transcribing the positiveshape to a resin to create a negative resin mold. That is to say, wheneither a positive or negative shape related to the present invention maybe processed by cutting, a prism sheet may be manufactured.

Manufacturing of the prism sheet according to the present invention willbe described in further details. A diamond tool bit having the sameshape as the cross-sectional shape of a prism structure according to thepresent invention is created and a negative mold is created byprocessing a 300 μ-thick copper-plated raw material by cutting to createa negative mold. The curved line provided by the present invention maybe approximated excellently with arcs. Further, instead of creating thediamond tool bit as aforementioned, a sward tool bit may be used to cutcurved portions of prism structures into groups of large numbers ofminute planer surfaces to create a positive mold. In this case, anegative mold used for forming plastic may be created by electroforming.

When the photopolymer method is used, an acrylic 2P resin (e.g., 30Y266,manufactured by Three-Bond Co., Ltd.) and an ultraviolet light sourcecan be used for exposure of about 1 J/cm² to harden the 2P resin and ashape of a metal mold is transcribed to the resin to create prisms onthe substrate. Further, injection molding and compression molding may beused appropriately in addition to the above-described methods. Stillfurther, a negative mold may be created by cutting a rolled raw materialof a metal mold may be cut by lathe processing to create a negative moldand applying the 2P resin onto a film of polyethylene terephthalate orother polymer material, preferably winded as a roll, to form a prismsheet. Furthermore, according to the present invention, a negative moldcreated by electroforming from a plane positive mold may be bended intoa roll and applied to continuous molding.

Summarization of the above description shows that the following effectsmay be obtained by the present invention. TABLE 2 Full width at halfFront brightness maximum Conventional 1 About 25 degrees prism Curvedprism 1.2 to 1.5 About 12 degrees

That is to say, as summarized in Table 2, the present invention mayprovide prism structures capable of deflecting light from a back lighthaving a directional launching property at an intensity peak of 60 to 80degrees and a full width at half maximum of not more than 20 degreesefficiently to the front side and condensing light at about half of afull width at half maximum in comparison to conventional prismstructures to improve the front brightness to 1.2 to 1.5 times.

Further, the present invention may provide the prism sheet having highefficiency of utilizing light, easiness of manufacturing and a functionto improve directionality of light

Still further, the present invention may provide the back-light unit andthe transmission type liquid crystal display device realizing highbrightness, high contrast, low power consumption, long battery life anda small size.

Furthermore, the present invention enables manufacturing prism sheetssufficiently capable of being produced in mass without a great increaseof a cost.

The present invention has so-far been described based on specificembodiments. However, the present invention is not limited to theabove-described specific embodiments. Further, regarding the presentinvention, the prism sheet used for the transmission type liquid crystaldisplay device has been described as particular embodiments. It isneedless to say however, that the present invention may be applied tooptical components used to deflect obliquely launched light efficientlyto the normal direction in addition to the prism sheet used for thetransmission type liquid crystal display device

1. A prism sheet for changing a direction of light propagation byreflecting light launched with an intensity distribution of apredetermined angle range, said prism sheet comprising: a reflectivesurface extending from a sheet base to a top for providing perfectreflection, said reflective surface comprising a higher order surfaceconstructed by at least a small curvature surface and a surfacedifferent from said small curvature surface.
 2. The prism sheetaccording to claim 1, wherein said higher order surface comprises aplurality of continuous planer surfaces.
 3. The prism sheet according toclaim 1, wherein said higher order surface comprises a planer surface asa said small curvature surface and a curved surface continuous thereto.4. The prism sheet according claim 1, wherein said prism sheet comprisesa plurality of said reflective surfaces formed in close proximity, andsaid each closely disposed reflective surface being connected through aconnection plane extending from said sheet base to said top of anadjacent reflective surface.
 5. The prism sheet according to claim 1,wherein said small curvature surface extends to a predetermined positionfrom said top of said reflective surface along with a direction towardsaid sheet base.
 6. The prism sheet according to claim 1, wherein saidconnection plane transmits said light launched with said intensitydistribution of said predetermined angle range to said reflectivesurface.
 7. The prism sheet according to claim 1, wherein atransmittance of said prism sheet is no less than 90%, and an angledistribution of exit light within 15 degrees at a full width at halfmaximum.
 8. A back-light unit used for a transmission type liquidcrystal display device comprising a liquid crystal panel, saidback-light unit comprising: a light source for providing light exposedto said liquid crystal panel, a planer light guide for changing adirection of propagation of said light to said liquid crystal panel, anda prism sheet disposed adjacently to said planer light guide; whereinsaid prism sheet comprises a reflective surface, extending from a sheetbase to a top for providing perfect reflection and said reflectivesurface comprises a higher order surface constructed by at least a smallcurvature surface and a surface different from said small curvaturesurface.
 9. The back-light unit according to claim 8, wherein saidhigher order surface comprises a plurality of continuous planersurfaces.
 10. The back-light unit according to claim 8, wherein saidhigher order surface comprises at least a planer surface as a said smallcurvature surface and a curved surface continuous thereto.
 11. Theback-light unit according to claim 8, wherein said prism sheet comprisesa plurality of said reflective surfaces formed in close proximity, andsaid each closely disposed reflective surface being connected through aconnection plane extending from said sheet base to said top of anadjacent reflective surface.
 12. The back-light unit according to claim8, wherein said small curvature surface extends to a predeterminedposition from said top of said reflective surface along with a directiontoward said sheet base.
 13. The back-light unit according to claim 8,wherein said connection plane transmits said light launched with saidintensity distribution of said predetermined angle range to saidreflective surface.
 14. The back-light unit according to claim 8,wherein a top of a reflective surface constituting said prism sheet isdisposed adjacently to said planer light guide.
 15. A transmission typeliquid crystal display device comprising a back-light unit and a liquidcrystal panel, said back-light unit comprising a light source forproviding light exposed to said liquid crystal panel, and a planer lightguide for changing a direction of propagation of said light to saidliquid crystal panel, and a prism sheet disposed adjacently to saidplaner light guide, wherein said prism sheet comprises a reflectivesurface extending from a sheet base to a top for providing perfectreflection and said reflective surface comprises a higher order surfaceconstructed by at least a small curvature surface and a surfacedifferent from said small curvature surface.
 16. The transmission typeliquid crystal display device according to claim 15, wherein said higherorder surface comprises a plurality of continuous planer surfaces. 17.The transmission type liquid crystal display device according to claim15, wherein said higher order surface comprises a planer surface as asaid small curvature surface and a curved surface continuous thereto.18. The transmission type liquid crystal display device according toclaim 15, wherein said prism sheet comprises a plurality of saidreflective surfaces formed in close proximity, and said each closelydisposed reflective surface being connected through a connection planeextending from said sheet base to said top of an adjacent reflectivesurface.
 19. The transmission type liquid crystal display deviceaccording to claim 15, wherein said small curvature surface extends to apredetermined position from said top of said reflective surface alongwith a direction toward said sheet base.
 20. The transmission typeliquid crystal display device according to claim 15, wherein saidconnection plane transmits said light launched with said intensitydistribution of said predetermined angle range to said reflectivesurface, and a top of a reflective surface constituting said prism sheetis disposed adjacently to said planer light guide.
 21. The transmissiontype liquid crystal display device according to claim 15, wherein a lenselement is further disposed between said back-light unit and said liquidcrystal panel.